The invention relates to a wet multi-disk clutch in part of a vehicle drive and a lubricant for a housing containing said clutch, comprising an input member and an output member, and a number of first disks connected to the input member and of second disks connected to the output member, which can be pressed together by actuating means.
Wet multi-disk clutches are used in drive trains of motor vehicles, predominantly in differential locks, in all-wheel drive systems for connecting up the additional drive axle or for changing the torque distribution between the axles.
In these applications, the clutch does not function simply as a selector clutch that can be engaged and disengaged but also as a control clutch and also operates for relatively long periods in slip. In this arrangement, vibrations occur, though sometimes only in the course of the clutch""s life, and these are perceived as anything from a low moan to a high-frequency whine. Apart from the fact that this noise is irritating, such vibrations lead to a reduction in service life.
The cause of these vibrations is probably a negative gradient in the friction coefficient. This gradient is taken to mean the reduction in the friction coefficient between the first and second disks as the rotational-speed difference increases. Initially, this is the transition from static friction to sliding friction with a relatively low coefficient of sliding friction; as the rotational-speed difference increases further, the friction coefficient can fall further.
The aim has therefore been to create a tribosystem in which the gradient of the friction coefficient is positive or at least not negative, at least in a certain range. One known solution to this is to choose pairs of disks in which the disks are composed of steel sheet and the first disks are provided with a sintered coating in certain surface patterns. Another known solution is to use fluids that are particularly suitable for wet clutches, as in automatic transmissions for example.
However, it has been found that even such tribosystems cannot reliably prevent the vibrations described, at least not after a certain time in operation, with the result that the disks soon have to be replaced. However, this is disadvantageous, especially in the case of clutches that are integrated into the vehicle drive because changing the disks is generally particularly labor-intensive.
When using such clutches in housings in which other drive components are also accommodated, another difficulty is that the drive components require oils with special lubricating properties, e.g. hypoid oils, but these are less suitable for clutches.
It is therefore the object of the invention to propose a tribosystem for friction clutches that prevents the occurrence of vibrations reliably and for the entire service life, even under the unfavorable conditions described.
According to the invention, this is achieved by virtue of the fact that the first disks are composed of steel and have on their friction faces linings composed of carbon fibers embedded in a carbon matrix, which linings are interrupted by channels leading from the inside outward, and that the second disks are composed of steel and their friction faces are provided with a nickel dispersion layer.
This tribosystem, which solves the problem set, has been determined in extensive tests. It has furthermore been found that the nickel dispersion layer is particularly effective if it has a crystalline structure and if it furthermore contains 2 to 5 percent by volume of uniformly dispersed titanium nitride.